Lab talk

Jul 20, 2014

Nanoscale position sensors: spintronics offer a low-cost alternative to optics

Position sensing with a subnanometre resolution has long been an exclusive domain of optical interferometry. It operates over bandwidths as high as a few hundred kHz and progress on alternative non-optical, non-contact sensors has been minimal. Reporting in Nanotechnology, scientists at IBM Research – Zurich introduce a new non-optical, non-contact position sensor. This is based on detecting changes in a high-gradient magnetic field of a microscale magnetic dipole by means of spintronic sensors.

In the sensor, the mechanical motion of a micromagnetic dipole is transduced into a change in electrical resistance of a spintronic-based magnetic field sensing element – for example, a giant magnetoresistance (GMR) sensor. Owing to the intrinsically high bandwidth and low measurement noise of these spintronic sensors, sensitivities of up to 40 Ohm/µm could be experimentally demonstrated. This leads to a noise floor of 0.5 pm/sqrt(Hz) over more than a megahertz bandwidth.

Increasing magnetic field gradient

Magnetoresistance-based position sensors have been long known in industrial mechatronics. However, their use in nanotechnology was limited due to a relatively low sensitivity and a large amount of hysteresis. In this new position-sensing concept, these issues are resolved by operating the spintronic sensor close to the pole of a micromagnetic dipole. This is where the magnetic field has an extremely high gradient, which increases as the dimensions of the micromagnet are scaled down.

Shape anisotropy

At the same time, saturation of the spintronic sensor is avoided due to the shape anisotropy effect. The fact that the positioning resolution improves as the sensor dimensions are scaled down sets the spintronic position sensor apart from most of the position sensors available today.

Readily useable

The position sensor has a low cost, a small form factor and low-complexity read-out electronics. This means that it can be readily used in a multitude of high-precision scientific tools and commercial instruments that require high-speed position sensing, such as atomic force microscopes. Its properties also make it highly suitable for multi-sensor configurations and applications where an easily adjustable trade-off between sensing resolution and range of operation is required.